| Abstract: | Commercial chitosan, and highly deacetylated congeners thereof, can be selectively and extensively sulfated at the O-6 level. Either a 2:1 mixture of sulfuric and chlorosulfonic acid (Method I) or a pyridine-SO3 complex, following protection of the 2-NH2 and 3-OH groups with copper ions (Method II), can be used. The properties of sulfated chitosan with different degrees of acetylation are studied at pH conditions which promote a polyampholyte structure. Light scattering and intrinsic viscosity (η]) measurements of chitosan 6-O-sulfate with a degree of acetylation of 20%, prepared by Method II, indicate the presence of molecular and structural heterogeneity, probably due to ordered structures in highly acetylated regions of the parent chitosan. This hypothesis is confirmed by the NMR analysis of the fragments obtained by depolymerization with nitrous acid. The heterogeneity is not observable in compounds obtained by Method I and in highly deacetylated chitosan 6-O-sulfate. The η] increase of the polyampholyte on decreasing ionic strength can be explained in terms of a more rigid conformation stabilized by electrostatic interactions between the sulfate groups and the protonated amino groups on neighbouring residues. |
